Calcium transients in neutrophils have been measured with the rapid mix flow cytometer. We have observed that the lag time for the response (2 seconds) is longer than the response time of the instrument (300 msecs). The ability to stimulate the cells uniformly has enabled us to detect cell populations in transition between minimal and maximal calcium levels. This project can be resumed with the new rapid mix device after addition of a second PMT to the rapid mix flow cytometer to make possible simultaneous analysis of ligand-receptor interactions and cell response.